PROBLEM 7.52 - Organic Chemistry

Question

Question: Why is the amine N atom more nucleophilic than the amide N atom  CH₃CONHCH₂CH₂CH₂NHCH₃?

Step-by-Step Solution

Answer

Answer 

The amide group is less nucleophilic as compared to the amine group in the given molecule. This is because the electron pair of the amide group is involved in resonance with the neighboring carbonyl group. 

Definition of a Nucleophile

A nucleophile is a chemical species in a reaction that can donate a pair of electrons.

The nucleophiles are classified as Lewis bases because of their ability to donate an electron pair.

 The examples of a nucleophile are given below:

• A molecule that has a lone pair of electrons such as NH₃ . In this case, the nitrogen atom has a lone pair of electrons, which makes it a nucleophile.
• A negatively charged ion such as chloride, bromide, etc.
• A bond can also act as a nucleophile.

Comparison of the nucleophilic nature of amines and amides

In the given molecule, the amine group, as well as the amide groups, are present. 

The nitrogen atom of the amine group is more electrophilic. This is because the lone pair of the nitrogen atom is localized on the nitrogen atom only, as shown in the following structure:

Representation of more nucleophilic center

On the other hand, in the case of amides, the lone pair of the nitrogen atom is delocalized with the carbonyl group. 

As a result of this delocalization, the electron density on the nitrogen atom decreases and makes it less nucleophilic, as  shown in the following structure:

Resonating structures

In one of the above resonating structures, the nitrogen atom has also acquired a positive charge. It makes the amides less nucleophilic as compared to amines.